For some applications, as for instance millimeter-wave wireless communication which is based in the 60 GHz band, over a distance of more than a few meters, high-gain antennas are required in order to overcome link budget limitations.
Such high gain antennas may be obtained with an antenna array comprising a plurality of antennas or antenna elements, which the radiation beams can be steered electronically. The signal on each of these antennas is modified by means of an amplifier/attenuator and phase shifter or delay line. For a dynamic beam forming, gains of amplifiers and phases of phase shifter can be controlled and adjusted. This can be modelled with complex numbers, called weights, whose modulus (or absolute value) represents the gain to apply to an amplifier dedicated to a considered antenna, and whose argument represent the phase shift to apply to the phase shifter. The beam forming permits to modify the sensitivity pattern of the antenna array, in order to make it more directional, i.e. to increase the sensitivity along a particular direction, where the signal is supposed to be, and/or to reduce the sensitivity along a different direction, for instance with interference.
For certain applications that require high antenna gain, one may employ antennas comprising many elements. However, having a high speed analogue-to-digital converter for each individual antenna signal is prohibitive in terms of power consumption and processing requirements for the base band processor. Therefore, it is preferable to use analog beam forming, i.e. a receiving station architecture in which the antenna signals are modified in the analogue domain (phase shifted and amplitude multiplied), added, and only then digitized with a single analogue-to-digital converter. In some applications, the receiving station comprises a plurality of analogue to digital converters, each one being common to a plurality of antenna elements (possibly few, M, analogue-to-digital converters, with M<N=number of antenna elements). Similarly, in the transmitting station, a single digital signal (or a few digital signals) are converted to analogue and split over multiple antenna elements. The analogue signals on each of the elements can be modified individually, just like in the receiver.
However, As the receiving station has fewer A/D converters (M) than antenna elements (N) it can only perform M channel measurements simultaneously, whereas a full channel measurement would require N measurements.